1. Field of the Invention
The present invention relates to a light emitting device and a method of manufacturing the same, and more particularly, to a light emitting diode having a plurality of nanocones formed at the surface thereof by epitaxial growth, whereby the light extraction efficiency of the light emitting diode is improved, and a method of manufacturing the same.
2. Discussion of the Related Art
Generally, a light emitting diode (LED) is a kind of semiconductor device that converts electricity into light using the characteristics of a compound semiconductor to transmit and receive a signal or is used as a light source. The light emitting diode generates high-efficiency light at low voltage with the result of high energy saving efficiency. Recently, the brightness of the light emitting diode, which was a limitation of the light emitting diode, has considerably improved, and therefore, the light emitting diode has been widely used throughout industry, such as backlight units, electric bulletin boards, display units, electric home appliances, and various kinds of automated equipment. Especially, a nitride light emitting diode has attracted considerable attention in the environmentally-friendly aspect because the energy band gap of an active layer constituting the nitride light emitting diode is wide with the result that light emitting spectrum is formed widely from ultraviolet rays to infrared rays, and the nitride light emitting diode does not contain environmentally hazardous materials, such as arsenic (As) and mercury (Hg).
In addition, research is being carried out on a light emitting diode having high brightness that is applicable in various applications. For example, a light emitting diode having high brightness may be obtained by improving the quality of an active layer of the light emitting diode to increase inner quantum efficiency or by assisting light generated from the active layer to be discharged to the outside and collecting the light in a desired direction to increase light extraction efficiency. Although attempts are being currently made to increase both the inner quantum efficiency and the light extraction efficiency, more active research is being carried out on a method of improving the electrode design, the shape, and the package of the light emitting diode to increase the light extraction efficiency than a method of improving the quality of a semiconductor material to increase the inner quantum efficiency.
Up to now, a method of increasing the transmissivity of an upper electrode of the light emitting diode or a method of disposing a reflection plate at the outside of the light emitting diode to gather light discharged to a sapphire substrate of the light emitting diode or the side of the light emitting diode upward has been mainly attempted. The light extraction efficiency is decided by the ratio of electrons injected into the light emitting diode to photons discharged from the light emitting diode. As the light extraction efficiency is increased, the brightness of the light emitting diode is increased. The light extraction efficiency of the light emitting diode is greatly affected by the shape or the surface state of a chip, the structure of the chip, and the package form of the chip. Consequently, it is necessary to pay careful attention when designing the light emitting diode.
For a light emitting diode with high output and high brightness, the light extraction efficiency acts as an important factor to decide the light emission efficiency of the light emitting diode. In a conventional method of manufacturing a nitride light emitting diode, however, the light extraction efficiency is limited.
FIG. 1 is a sectional view illustrating a conventional nitride light emitting diode. As shown in FIG. 1, the conventional nitride light emitting diode is constructed in a structure in which a buffer layer 11, an n-type nitride semiconductor layer 12, an active layer 13, and a p-type nitride semiconductor layer 14 are sequentially stacked on a sapphire substrate 10. Mesa etching is carried out from the p-type nitride semiconductor layer 14 to a portion of the n-type nitride semiconductor layer 12. As a result, the etched portion of the n-type nitride semiconductor layer 12 is exposed to the outside. An n-electrode 15 is formed on the exposed portion of the n-type nitride semiconductor layer 12. Also, a transparent electrode 16 is formed on the p-type nitride semiconductor layer 14, and a p-electrode 17 is formed on the transparent electrode 16.
A method of manufacturing the nitride light emitting diode is carried out as follows. First, a buffer layer 11, an n-type nitride semiconductor layer 12, an active layer 13, and a p-type nitride semiconductor layer 14 are sequentially formed on a sapphire substrate 10. Subsequently, mesa etching is carried out from the p-type nitride semiconductor layer 14 to a portion of the n-type nitride semiconductor layer 12 using a reactive ion etching (RIE) method. A transparent electrode 16 to improve the ohmic characteristics is formed on the p-type nitride semiconductor layer 14, and a p-electrode 17 is formed on the transparent electrode 16. Subsequently, an n-electrode 15 is formed on the mesa etched and thus exposed portion of the n-type nitride semiconductor layer.
The light emitting diode is driven as follows. When voltage is applied to the p-electrode 17 and the n-electrode 15, holes and electrons move from the p-type nitride semiconductor layer 14 and the n-type nitride semiconductor layer 12 to the active layer 13. The electrons and the holes are recoupled with each other in the active layer 13, whereby light is generated from the active layer 13. The light generated from the active layer 13 advances upward and downward from the active layer 13. The upward-advancing light is discharged to the outside through the transparent electrode 16 thinly formed on the p-type nitride semiconductor layer 14. On the other hand, the downward-advancing light is discharged downward through the substrate 10, and is then absorbed into solder used when packaging the light emitting diode, or else, the downward-advancing light is reflected by the substrate 10, moves upward, and is then reabsorbed into the active layer 13, or is discharged to the outside through the transparent electrode 16.
In the conventional nitride light emitting diode, however, a total reflection condition occurs due to the difference in a refractive index between a nitride semiconductor material and the outside when light generated from the active layer is discharged to the outside. As a result, light incident at an angle greater than the critical angle of the total reflection is not discharged to the outside but is reflected into the light emitting diode. Specifically, as shown in FIG. 2, when light generated from an active layer 30 reaches the surface of a nitride semiconductor material 40, the light is not discharged to the outside but is reflected into the light emitting diode if the incident angle of the incident light exceeds the critical angle θc, which is decided by the outer refractive index and the refractive index of the nitride semiconductor material. The reflected light is diminished as the light passes through several channels.
The critical angle is decided by Snell's Law. Specifically, the critical angle may be obtained by the following equation.sin θc=N1/N2   [Equation 1]
Where, θc is the critical angle, N1 is the outer refractive index of the light emitting diode, and N2 is the inner refractive index of the light emitting diode.
When the light generated from the active layer reaches the surface of the nitride semiconductor material as described above, the light is totally reflected into the light emitting diode and thus diminished, whereby the light extraction efficiency of the conventional nitride light emitting diode is lowered.
In order to solve the above-mentioned problem, there has been proposed a method of etching the surface of the light emitting diode to rough the surface of the light emitting diode. However, the etching process is further performed after the growth of the thin film on the light emitting diode, whereby the method of manufacturing the light emitting diode is complicated, and therefore, the time necessary for manufacturing the light emitting diode is increased. In addition, a conventional light emitting device package structure is manufactured by plastic injection molding, and therefore, the miniaturization and thin shaping possibilities of the light emitting device package structure are limited. Consequently, the conventional light emitting device package structure is not suitable for a current tendency requiring the reduction in weight and size of electronic products.